Ocular procedures often modify one or more structures of the eye, such as the cornea, lens or retina. Some procedures involve removing or replacing one or more structures of the eye, or adding an implant. For example, lens replacement surgery involves removing a patient's existing lens and replacing it with a new lens. Some procedures, such as laser vision correction surgery, do not remove or replace existing structures of a patient's eye, or add an implant to the eye, but rather reshape existing structures. Regardless of the type of modification being made (e.g. removal, replacement, insertion, or alteration), the optical performance of the eye is altered by adjustments made to the structures of the eye. Therefore in order to accurately model the structure of any eye, it is necessary to determine the ocular parameters of that eye. These parameters include shape, thickness and refractive index of ocular structures such as the cornea, the lens, the retina, or any other structure of interest.
Measuring parameters such as curvatures, or shapes of surfaces, or thickness of elements within a patient's eye is traditionally carried out using variations of ultrasound Optical Coherence Tomography (OCT) or Optical Low Coherence Reflectometry (OLCR), Purkinje or Scheimpflug imaging systems.
With living subjects and taking into account the precision of the measurements required it is necessary to introduce a level of movement compensation. Solutions for movement compensation are introduced in US 2012140175 A (CARL ZEISS MEDITEC, INC) 24 Jan. 2012, US 2013188140 A (CARL ZEISS, MEDITEC, INC) 17 Jan. 2013, US 2011267340 A (UNIV FRIEDRICH ALEXANDER ER [DE]; MASSACHUSETTS INST TECHNOLOGY [US]) 29 Apr. 2011, EP 2198771 A (OPTOPOL TECHNOLOGY, SA) 2 Dec. 2012, and WO 2010/101162 A (CANON KK [JP]; NUMAJIRI YASUYUKI [JP]; YAMADA KAZURO [JP]; HIROSE FUTOSHI [JP) 24 Feb. 2010. Additionally, US 2012249956 A (NARASIMHA-IYER HARIHAR [US]; EVERETT MATTHEW J [US]; ZEISS CARL MEDITEC INC [US]), WO 2010/149420 A (SIEMENS AG [DE]; KLEINFELD JENS [DE]; KLUGE ANDRE [DE]; MEISL JUERGEN [DE]; TUESCHEN SABINE [DE]; WO) 26 Mar. 2009 US 2013195336 A (UCHIDA HIROKI [JP]; CANON KK [JP) 30 Aug. 2012 apply additional data processing and scan patterns to movement compensation to account for movement of the subject/patient's head. US 2009091766 A (CANON KK [JP]) 30 Aug. 2008 and DE 102009022958 A (ZEISS CARL MEDITEC AG [DE]) 28 May 2009 also acquire simultaneous fundus images in order to track eye movement and thus compensate for same in order to improve the determination of optical parameters.
To further improve the determination and accuracy of optical measurements using OCT/OLCR, attempts have been made to combine measurements made using these systems with additional measurement/movement detection to correct motion artefacts. Advances have been made in corneal vertex position detection through the use of additional measuring modality to provide accurate axial length measurement.
U.S. Pat. No. 5,387,951 B (TOPCON CORP) 7 Mar. 1995 discloses an apparatus for intraocular length measurements which avoids errors in measurement caused by the movement of a subjects head. By concurrently measuring the axial movement of the eye during the measurement process, the actual eye length can be calculated by adjusting the apparent length by the amount of axial movement. A disadvantage of this system is that motion correction applies only to axial length measurement.
US 2007076217 (CARL ZEISS MEDITEC, INC) 5 Oct. 2005 is directed to the use of optical coherence tomography for eye length measurement. A second OCT device is used to correct measurements made by the first OCT device. As with U.S. Pat. No. 5,387,951 this disclosure is only directed to axial length correction rather than a total A-scan. Furthermore, only z-axis displacement is accounted for in measurements and corrections.
US 2010014051 A (SIS AG SURGICAL INSTR SYSTEMS) 24 Jul. 2008 combined interferometric techniques to determine a relative position of the retina of the eye and uses a second non-interferometric technique to determine a relative position of the cornea and uses these measurements to determine the axial length of the eye. As previously, only z-axis displacement is accounted for. No motion compensation A-scan measurements are possible.
WO 2014/043517 A (UNIV JOHNS HOPKINS [US]) 13 Sep. 2013 is also directed to a motion compensated OCT and feedback system which provides for correction of deviation based on cross correlation of adjacent scans. However, this document does not provide for absolute motion tracking.
WO 2010/010116 A (BIOGASOL IPR APS [DK]; MIKKELSEN MARIE JUST [DK]; YAO SHUO [DK]) 29 Jul. 2009 implements the Scheimpflug principles in corneal vertex position detection with additional measuring modality. This document provides a means for measuring corneal vertex position (x, y, z) however, provides only for axial eye motion, i.e. z-axis compensation.
WO 2009/149953 A (ZEISS CARL MEDITEC AG [DE]; BUBLITZ DANIEL [DE]; KRAMPERT GERHARD [DE]; HACKER MARTIN [DE]) 12 Jun. 2009 uses first and second OCT measurements to compensate for a complete A-scan OCT, but again only provides for active eye motion compensation, is z-axis compensation.
It is therefore an object of the present invention to implement an improved compensation system and method for eye movement compensated measurements of multiple ocular surface locations. In systems such as those described above where two methods of measurement are implemented compensation is only for axial length measurements and compensation is for z-axis displacement. It is an object of the present invention to provide a more accurate system and namely a measurement system that is not influenced by eye motions.